System and method for multiple mode flexible excitation and acoustic chaos in sonic infrared imaging

ABSTRACT

A defect detection system for thermally imaging a structure that has been energized by a sound energy. The system includes a transducer that couples a sound signal into the structure, where the sound signal causes defects in the structure to heat up. In one embodiment, the sound signal has one or more frequencies that are at or near an eigen-mode of the structure. In another embodiment, an on-linear coupling material is positioned between the transducer and the structure to couple the sound energy from the transducer to the structure. A predetermined force is applied to the transducer and a pulse duration and a pulse frequency of the sound signal are selected so that the sound energy induces acoustic chaos in the structure, thus generating increased thermal energy. A thermal imaging camera images the structure when it is heated by the sound signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. ProvisionalApplication No. 60/453,431, titled System and Method for Acoustic Chaosand Sonic Infrared Imaging, filed Mar. 10, 2003 and U.S. ProvisionalApplication No. 60/407,207, titled System and Method for Acoustic Chaosand Sonic Infrared Imaging, filed Aug. 28, 2002.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates generally to a system and associatedmethod for detecting defects in a material and, more particularly, to asystem and associated method for detecting defects in a material, wherethe system includes a transducer for coupling sound energy into thematerial in a manner that creates acoustic chaos in the material or forcoupling a multiple mode flexible excitation signal into the material,and includes a thermal imaging camera for imaging the heat created inthe material as a result of the acoustic chaos or flexible excitation.

[0004] 2. Discussion of the Related Art

[0005] Maintaining the structural integrity of certain structures isvery important in many fields because of safety concerns, downtime,cost, etc. Loss of structural integrity is typically caused by materialdefects, such as cracks, delaminations, disbonds, corrosion, inclusions,voids, etc., that may exist in the structure. For example, it is veryimportant in the power generation industry that reliable techniques areavailable to examine the structural integrity of turbine, generator andassociated balance of plant equipment to ensure the components andsystems do not suffer failure during operation. Similarly, it is veryimportant in the aviation industry that reliable techniques areavailable to examine the structural integrity of the aircraft skin andstructural components of the aircraft to ensure that the aircraft doesnot suffer structural failure when in flight. The structural integrityof turbine blades and rotors and vehicle cylinder heads is also veryimportant in those industries. The most common method for detection of acrack or defect is visual examination by skilled personnel. But, it isknown that cracks or defects that may affect the integrity of structuralcomponents may not be readily visible without the use of specialtechniques to aid the examiner. Therefore, various techniques have beendeveloped in the art for the non-invasive and non-destructive analysisof different structural components and materials in many industries.

[0006] One known technique for the non-invasive and non-destructivetesting of a material for defects includes treating the material with adye penetrant so that the dye enters any crack or defect that may bepresent in the material. The material is then cleaned and treated with apowder that causes the dye that remains in the crack to wick into thepowder. An ultraviolet (UV) light source is used to inspect the materialto observe locations in the material that fluoresce as a result of thedye. This technique has the disadvantage, however, that it is highlyinspector intensive and dependent because the person inspecting for thefluorescence must be skilled. Additionally, the dye does not penetratetightly closed cracks or cracks that are not on the surface of thematerial.

[0007] A second known technique for inspecting a component for defectsemploys an electromagnetic coil to induce eddy currents in thecomponent. The coil is moved around on the component, and the eddycurrent pattern changes at a crack or other defect. The compleximpedance in the coil changes as the eddy current changes, which can beobserved on an oscilloscope. This technique has the drawback, however,that it is also very operator intensive, and is also extremely slow andtedious.

[0008] Another known technique for detecting defects in a componentemploys thermal imaging of the component to identify the defects. Inother thermal imaging techniques, a heat source, such as a flash lamp ora heat gun, is used to direct a planar pulse of heat to the surface ofthe component. The component absorbs the heat, and emits radiation inthe infrared wavelengths. Certain types of defects will cause thesurface temperature to cool at a different rate around the defect thanfor the surface temperature of surrounding areas. A thermal or infraredimaging camera is used to image the component and detect the resultingsurface temperature variations. Although this technique has beensuccessful for detecting disbonds and corrosions, it is ordinarily notsuccessful for detecting vertical cracks in the component, that is,those cracks that are perpendicular to the surface of the component.This is because a fatigue crack looks like a knife edge to the planarheat pulse, and therefore no, or minimal, heat reflections occur fromthe crack making it difficult or impossible to see in a thermal image.

[0009] Thermal imaging for detecting defects in a material has beenextended to systems that employ ultrasonic excitation of the material togenerate the heat. An acoustic thermal effect occurs when sound wavespropagate through a solid body that contains a crack or other defectcausing it to vibrate. Because the faces of the crack ordinarily do notvibrate in unison as the sound waves pass, dissipative phenomena, suchas friction between the faces, will convert some of the vibrationalenergy to heat. By combining this heating effect with infrared imaging,a very efficient, non-destructive crack detection system can berealized. Such imaging systems are generally described in the literatureas sonic IR, thermosonic, acoustic thermography, etc.

[0010] The article Rantala, J., et al. “Lock-in Thermography withMechanical Loss Angle Heating at Ultrasonic Frequencies,” QuantitativeInfrared Thermography, Eurotherm Series 50, Edizioni Ets Piza 1997, pgs.389-393 discloses such a defect detection technique. The ultrasonicwaves cause the opposing edges of the crack to rub together causing thecrack to heat up. Because the undamaged part of the component is onlyminimally heated by the ultrasonic waves, the resulting thermal imagesof the component show the crack as a bright area against a darkbackground field.

[0011] U.S. Pat. No. 6,236,049 issued May 22, 2001 to Thomas et al.titled “Infrared Imaging of Ultrasonically Excited Subsurface Defects inMaterials,” assigned to the Assignee of this application, and hereinincorporated by reference, discloses a thermal imaging system fordetecting cracks and other defects in a component by ultrasonicexcitation. An ultrasonic transducer is coupled to the component, andultrasonic energy from the transducer causes the defects to heat up,which is detected by a thermal camera. The ultrasonic energy is in theform of a substantially constant amplitude pulse. A control unit isemployed to provide timing and control functions for the operation ofthe ultrasonic transducer and the camera.

SUMMARY OF THE INVENTION

[0012] In accordance with the teachings of the present invention, asystem and method is disclosed for thermal imaging subsurface cracks andother defects in a structure that have been heated by sound energy. Asound source, such as a transducer, couples a sound signal into thestructure, where the sound waves in the signal cause the edges of thedefects to vibrate against each other and heat up. A thermal imagingcamera images the structure when it is being heated by the sound sourceto identify the defects.

[0013] In one embodiment, the sound signal includes a combination offrequencies selected for the particular structure so that the frequencyoccurs or doesn't occur at or near an eigen-mode of the structure. Inthis embodiment, the sound source can generate a combination of signalscentered at different frequencies, a chirp-signal, a pulse-envelopesignal, etc., to vary the sound signal in frequency, amplitude andduration so that the eigen-mode is excited or avoided for flexiblemultiple mode excitation.

[0014] In another embodiment, a coupling material, such as a non-linearcoupling material, is positioned between the sound source and thestructure to couple the sound signal into the structure. A predeterminedforce is applied to the sound source to push it against the structure.The force and the pulse duration and frequency of the sound signal areselected so that the sound energy induces acoustic chaos in thestructure, where the acoustic chaos increases the generation of thermalenergy. The vibration of the structure can be measured by a vibrometeror microphone to determine if chaos frequencies are present.

[0015] Additional advantages and features of the present invention willbecome apparent from the following description and appended claims,taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a block diagram of a defect detection system, accordingto an embodiment of the present invention;

[0017]FIG. 2 is a broken-away, side view of a portion of the defectdetecting system shown in FIG. 1;

[0018] FIGS. 3(A)-3(D) show consecutive images at predetermined timeintervals of an open crack in a component that has been ultrasonicallyexcited and thermally imaged by the defect detection system of thepresent invention;

[0019]FIG. 4 is a plan view of a defect detection system employing anelectromagnetic acoustic transducer, according to another embodiment ofthe present invention;

[0020]FIG. 5 is a waveform showing the vibrational response of a samplethat has been excited by a 40 kHz excitation pulse, where the waveformhas been separated into five regions A-E;

[0021]FIG. 6 is a graph with frequency on the horizontal axis andamplitude on the vertical axis showing the frequency peaks generated byacoustic chaos in region D of the waveform shown in FIG. 5;

[0022]FIG. 7 is a graph with frequency on the horizontal axis andamplitude on the vertical axis showing the frequency peaks generated byacoustic chaos in region E of the waveform shown in FIG. 5;

[0023]FIG. 8 is a block diagram of an acoustic chaos defect detectionsystem, according to another embodiment of the present invention;

[0024]FIG. 9 is a block diagram of a thermography defect detectionsystem, according to another embodiment of the present invention, thatis able to provide a flexible multiple mode input signal having selectedfrequencies to control the frequency, amplitude and duration of theinput signal;

[0025]FIG. 10 is a graph with time on the horizontal axis and amplitudeon the vertical axis showing part of an input excitation signal for thesystem shown in FIG. 9 that has two frequencies, where a first frequencyis centered at 20 kHz and a second frequency is centered at 21 kHz;

[0026]FIG. 11 is a graph with time on the horizontal axis and amplitudeon the vertical axis showing part of an input excitation signal for thesystem shown in FIG. 9 that has two frequencies, where a first frequencyis centered at 20 kHz and a second frequency is centered at 40.5 kHz;

[0027]FIG. 12 is a graph with time on the horizontal axis and amplitudeon the vertical axis showing part of an input excitation signal for thesystem shown in FIG. 9 that has two frequencies, where one frequency iscentered at 20 kHz and the other frequency is centered at 41 kHz;

[0028]FIG. 13 is a graph with time on the horizontal axis and amplitudeon the vertical axis showing part of an input excitation signal for thesystem shown in FIG. 9 that has three frequencies, where a firstfrequency is centered at 20 kHz, a second frequency is centered at 21kHz and a third frequency is centered at 22 kHz;

[0029]FIG. 14 is a graph with time on the horizontal axis and amplitudeon the vertical axis showing an input excitation signal for the systemshown in FIG. 9 that is a Gaussian frequency band around 20 kHz;

[0030]FIG. 15 is a graph with time on the horizontal axis and amplitudeon the vertical axis showing an input excitation signal for the systemshown in FIG. 9 that is a chirp-signal swept upwards;

[0031]FIG. 16 is a graph with time on the horizontal axis and amplitudeon the vertical axis showing an input excitation signal for the systemshown in FIG. 9 that is a signature signal having random pulses in adigital sequence;

[0032]FIG. 17 is a graph with time on the horizontal axis and amplitudeon the vertical axis showing an input excitation signal for the systemshown in FIG. 9 that is based on a rectangular frequency band centeredaround 20 kHz;

[0033]FIG. 18 is a graph with time on the horizontal axis and amplitudeon the vertical axis showing an input excitation signal for the systemshown in FIG. 9 that has an increasing amplitude with a step at thebeginning; and

[0034]FIG. 19 is a graph with time on the horizontal axis and amplitudeon the vertical axis showing an input excitation signal for the systemshown in FIG. 9 that includes two pulses each with a favored envelopefrequency.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0035] The following description of the embodiments of the inventiondirected to a defect detection system for detecting defects in astructure is merely exemplary in nature, and is in no way intended tolimit the invention or its applications or uses.

[0036]FIG. 1 is a block diagram of a defect detection system 10,according to an embodiment of the present invention. The system 10 isbeing used to detect defects, such as cracks, corrosion, delaminations,disbonds, etc., in a component 12. The component 12 is intended torepresent any structural component or material, such as an aircraftskin, turbine blade, turbine rotor, power generator, vehicle cylinderhead, etc., that may include these types of defects that could causecatastrophic failure. It is stressed that the component 12 does not needto be metal, but can be other materials, such as ceramics, composites,etc.

[0037] The system 10 includes an ultrasonic transducer 14 that generatesa sound signal within a certain ultrasonic frequency band. Theultrasonic transducer 14 includes a horn 18 that couples the soundsignal into the component 12. The transducer 14 can be a conventionaltransducer suitable for the purposes of the thermosonic process of thepresent invention. The transducer 14 provides a transformation ofelectrical pulses into mechanical displacement by use of a piezoelectricelement. For example, the transducer 14 may employ a PZT stack ofpiezoelectric crystals that are cut to precise dimensions and operate ata very narrow frequency as dictated by the cut dimension of thecrystals. The PZT stack is mechanically coupled to the horn 18, and thetip of the horn 18 is pressed against the component 12. Because the tiphas a fixed dimension and is inflexible, it exhibits a wide contact areaand pressure within the area of contact. This is further influenced by anon-flat, non-smooth surface of the component 12. The transducer 14 canalso be a tunable piezo-mechanical exciter, such as those described inU.S. Pat. Nos. 6,232,701 and 6,274,967, or the model F7-1 piezoelectricshaker system manufactured by Wilcox Research of Gaithersburg, Md.

[0038] In one embodiment, the transducer 14 generates pulses ofultrasonic energy at a frequency of about 40 kHz for a period of time ofabout ½ of a second and a power level of a bout 800 watts. However, aswill be appreciated by those skilled in the art, other ultrasonic orsonic frequencies, power levels and pulse durations can be used withinthe scope of the present invention. The transducer 14 can be the 800 WBranson 40 kHz power supply driving an ultrasonic welding transducer.

[0039] The ultrasonic energy from the transducer 14 is coupled into thecomponent 12 through a mechanical coupler 16. The coupler 16 is inmechanical contact with the transducer horn 18 and a front side 20 ofthe component 12. FIG. 2 is broken-away, side view showing the horn 18in contact with the coupler 16 and the component 12. In one embodiment,the coupler 16 is a non-linear coupler, such as an automotive gasketmaterial, leather, duct tape, cork, Teflon, paper, etc., that helpscreate acoustic chaos, discussed below, within the component 12 aroundthe defect as a result of the acoustic energy. In other embodiments, thecoupler 16 can be a thin piece of a soft metal, such as copper, toeffectively couple the ultrasonic energy into the component 12. It isnoted, however, that the coupler 16 may not be required in certainapplications, and yet still provide acoustic chaos. A force 26 isapplied to the transducer 14 by any suitable device (not shown) to pushthe horn 18 against the coupler 16 and the component 12. The amount ofthe force 26 applied to the transducer 14 is selected to further enhancethe generation of acoustic chaos within the component 12.

[0040] The detection system 10 includes a thermal imaging camera 22spaced a predetermined distance from the component 12, as shown. Thecamera 22 generates images of the component 12 in conjunction with theultrasonic excitation of the component 12. The camera 22 can be spacedfrom a back side 24 of the component 12 at any distance that is suitableto provide images of as much of the component 12 as desired in a singleimage to simultaneously detect multiple defects with the desiredresolution. In other embodiments, the ultrasonic energy from thetransducer 14 and the image generated by the camera 22 can be providedat the same side of the component 12 or any side of the component 12.The thermal camera 22 can be any camera suitable for the purposesdescribed herein, such as the Radiance HS camera available from Raytheonor the Indigo Systems Phoenix IR camera. In one embodiment, the camera22 senses infrared emissions in a 3-5 micron wavelength range, andgenerates images at 100 frames per second. The camera 22 may include afocal plane array having 256×256 InSb pixels to generate the desirableresolution.

[0041] A controller 30 provides timing between the transducer 14 and thecamera 22. The controller 30 can be any computer suitable for thepurposes described herein. When the detection process is initiated, thecontroller 30 causes the camera 22 to begin taking sequential images ofthe component 12 at a predetermined rate. Once the sequence of imagesbegins, the controller 30 sends a signal to a power amplifier 32 thatcauses the amplifier 32 to send a pulse to the transducer 14 to generatethe pulsed ultrasonic signal. The ultrasonic energy is in the form of asimple pulse at the desired frequency. The image is generated by thecamera 22 and sent to a monitor 34 that displays the images of thecomponent 12. The images can also be sent to a storage device 36 to beviewed at another location if desirable.

[0042] The ultrasonic energy applied to the component 12 causes thefaces of cracks and other defects in the component 12 to rub againsteach other and create heat. By providing the proper parameters in thesystem 10, as discussed herein, acoustic chaos is created in thecomponent 12 to enhance the heating of the defect. The heat appears asbright spots in the images generated by the camera 22. Therefore, thesystem 10 is good at identifying very small tightly closed cracks. Forthose cracks that may be open, where the faces of the crack do nottouch, the heat is generated at the stress concentration point at thecrack tip. This point appears as a bright spot on the images indicatingthe end or tip of an open crack. The ultrasonic energy is effective toheat the crack or defect in the component 12 regardless of theorientation of the crack relative to the energy pulse. The camera 22takes an image of the surface of the component 12 providing a visualindication of any crack in the component 12 no matter what the positionof the crack within the thickness of the component 12.

[0043] As will be discussed in more detail below, the ultrasonic energyfrom the transducer 14 generates acoustic chaos in the component 12. Theacoustic chaos can be measured by measuring the vibration of thecomponent 12 to determine the chaos frequencies. In one embodiment, avibrometer 28, such as the Polytec PI OFV-511 single fiber Doppler laservibrometer, can be used to measure the vibrations of the component 12.The vibrometer 28 emits an optical beam towards the component 12, andoptical reflections therefrom are received by the vibrometer 28. Thetime of travel of the optical signal to the component 12 and backdetermines how close the component 12 is to the vibrometer 28, and thusits vibration. The vibrometer 28 uses the doppler effect and suitablealgorithms to calculate the vibration frequencies. The measurements madeby the vibrometer 28 are sent to the controller 30 and displayed asfrequency signals on the monitor 34. The controller 30 Fouriertransforms the signals from the vibrometer 28 to generate the frequencysignals that are time dependent on the vibration spectra. In oneembodiment, the vibrometer 28 has a digitizing rate up to 2.56 MHz, sothat vibrational frequencies up to about 1.2 MHz can be determined. Itis noted that the vibrometer 28 does not necessarily have to be aimednormally at the component 12.

[0044] In an alternate embodiment, the vibrometer 28 can be replacedwith a microphone that simply measures the audible frequencies, or thehorn “screech,” when the transducer 14 emits the ultrasonic pulse. It isbelieved that the horn screech itself is an indication that acousticchaos is occurring in the component 12. The signals received by themicrophone are also sent to the controller to be displayed on themonitor 34.

[0045] To illustrate the process of imaging a crack in a component asdiscussed herein, FIGS. 3(A)-3(D) show four sequential images 38 of anopen fatigue crack 40 in a structure 42. FIG. 3(A) shows the image 38 ofthe structure 42 prior to the ultrasonic energy being applied. FIG. 3(B)shows the image 38 of the structure 42 about 14 ms after the ultrasonicenergy is applied. As is apparent, a light (higher temperature) spot 44(sketched as a dark region) appears at the closed end of the crack 40,where mechanical agitation causes the heating. FIGS. 3(C) and 3(D) showsubsequent images 38 at times of about 64 ms and 114 ms, respectively.The light spot 44 on the image 38 increases dramatically over thissequence, clearly indicating the location of the crack 40.

[0046] According to another embodiment of the present invention, thetransducer 14 can be replaced with an electromagnetic acoustictransducer (EMAT). An EMAT used for this purpose is disclosed in U.S.Pat. No. 6,399,948 issued to Thomas et al., assigned to Wayne StateUniversity and Siemens Westinghouse Power Corporation, and hereinincorporated by reference.

[0047] An EMAT includes a permanent magnet, or electromagnet, thatgenerates a static magnetic field in the object being tested. Anelectromagnet is provided that would be energized with a time-varyingcurrent to generate eddy currents on and just beneath the surface of theobject being tested. The eddy currents interact with the static magneticfield to generate a Lorentz force that acts on free electrons in theobject, which induce collisions with ions in the object in a directionmutually perpendicular to the direction of the static magnetic field andthe local eddy currents. This interaction generates sound waves ofvarious polarizations that are reflected off of discontinuities in theobject to identify defects. In the present invention, these sound wavesgenerate heat at the defect site. The sound waves can be in variousforms, including, but not limited to sheer waves, surface waves, platewaves, Raleigh waves, lamb waves, etc. In order to generate the acousticchaos as discussed herein and transmit a chaotic waveform, the EMATcannot be tuned to a specific resonant frequency, but should bebroadband.

[0048] To illustrate this embodiment of the present invention, FIG. 4 isa broken-away, perspective view of a defect detection system 50employing an EMAT 52 of the type discussed above. The EMAT 52 ispositioned against a turbine blade 54 inside of a turbine engine, butcan be any suitable part being detected for defects. A length of cable56 is coupled to the EMAT 52 and a controller (not shown), such as thecontroller 30 above. The cable 56 includes a coil 58 wrapped around apermanent magnet 60. An AC voltage signal on the cable 56 applied to thecoil 58 causes eddy currents to interact with the static magnetic fieldgenerated by the permanent magnet 60 in the turbine blade 54. Theinteraction of the eddy currents and the static magnetic field generatessonic or ultrasonic waves that cause the faces of a crack 62 in theblade 54, or other defect, to rub against each other and generate heatradiation 64. A radiation-collecting device 66 is coupled to a suitableinfrared camera (not shown), such as the camera 22, to provide theimages.

[0049] A coupling material may be provided between the permanent magnet60 and the turbine blade 54 to effectively couple the electromagneticenergy from the EMAT 52 into the turbine blade 54. The coupling materialcould be part of the permanent structure of the magnet 60 to make thesystem 50 more applicable for remote detection inside of a turbineengine. Because the EMAT 52 can be made broadband, the chaos would becreated in the turbine blade 54 by applying an electrically generatedchaos signal as discussed below.

[0050] According to the invention, acoustic chaos is created in thecomponent 12, which acts to increase the amount of thermal energy at thedefect in the component 12 above that which would be generated in theabsence of acoustic chaos. Acoustic chaos is defined herein as a rangeof frequencies providing a vibrational waveform whose spectralfrequencies are related to the excitation frequency (here 40 kHz) by theratios of rational numbers. The frequencies associated with acousticalchaos can be both lower and higher than the excitation frequency.Acoustic chaos can be modeled as a mathematical relationship, and hasbeen well documented in the literature. One such example can be found inRasband, S. Neil, et al., “Chaotic Dynamics of Non-Linear Systems,”(1990).

[0051] To generate acoustic chaos in the component 12, the correctcombination of the force 26 applied to the transducer 14, the materialof the coupler 16, the thickness of the coupler 16, the frequency of theacoustic input pulse and the duration of the acoustic input pulse mustbe provided. A 40 kHz acoustic pulse is beyond normal adult hearing.However, it has been observed that the best image quality from thecamera 22 occurs if an acoustic sound, or “horn screech” is sensed. Thepresence of this audible screech is ordinarily attributed tonon-linearities in the coupling between the horn 18 and the component12. It has been discovered, however, that this horn screech occurs as aresult of anharmonic frequencies resulting from the onset of acousticchaos or from pseudo-chaotic conditions that precede acoustic chaos.

[0052] Various materials that exhibit non-linear characteristics aresuitable for the coupler 16. The coupler 16 is compressed by the force26 applied to the transducer 14 to keep the horn 18 in place against thecomponent 12, and provide a tight contact. However, it has been observedthat the amount of the force 26 applied to the transducer 14 helpsobtain the desired screech, and thus a higher quality image. If theforce is too little, then very little sound is coupled into thecomponent 12. The same affect occurs if the force 26 is too great. Theexact amount of force necessary to produce the screech depends upon theparticular acoustic horn being used to inject the sound, presumablybecause different horns have different vibration amplitudes. Thus, aparticular combination of vibration amplitude and applied force iscrucial to generating the screech.

[0053] It is possible that the proper force applied to the horn 18 willallow the tip of the horn 18 to recoil from the surface of the component12 during the negative half of the acoustic period of the input pulse.If such a recoil occurs, the input to the component 12 will be more likea series of equally spaced kicks or bumps at the ultrasonic inputfrequency, than a sinusoidal wave. When the system being kicked hasnatural resonances, it is likely that one or more of these resonanceswill be excited by the kicks. The solution of the mathematical problemof a resonant system that is subject to a series of regularly spacedkicks can be found in the book referenced above. After the nth kick, thesolution is:

X _(n) =A _(n) cos ωnτ+B _(n) sin ωnτ,  (1)

[0054] where the coefficients A_(n) and B_(n) are given by:$\begin{matrix}{A_{n} = {A_{1} + {\frac{C}{\omega}\sin \quad \pi \quad {{n\left( \frac{\omega}{\Omega} \right)}\left\lbrack {{\cos \quad \pi \quad {n\left( \frac{\omega}{\Omega} \right)}} - {\cot \quad \pi \quad {n\left( \frac{\omega}{\Omega} \right)}\sin \quad \pi \quad {n\left( \frac{\omega}{\Omega} \right)}}} \right\rbrack}}}} & (2) \\{B_{n} = {B_{1} + {\frac{C}{\omega}\left\lbrack {{\sin \quad \pi \quad {{n\left( \frac{\omega}{\Omega} \right)}\left\lbrack {{\sin \quad \pi \quad {\eta \left( \frac{\omega}{\Omega} \right)}} + {\cot \quad \pi \quad {n\left( \frac{\omega}{\Omega} \right)}\cos \quad \pi \quad {n\left( \frac{\omega}{\Omega} \right)}}} \right\rbrack}} - 1} \right\rbrack}}} & (3)\end{matrix}$

[0055] Here, C is the strength of the kick, ω is the natural frequencyof the oscillator, and Ω=(2π/τ) is the angular “kicking” frequency. Whenω/Ω is a rational fraction, this set of equations is periodic and thepossibility of a resonance exists.

[0056] To further study the occurrence of acoustic chaos in thecomponent 12 as a result of the application of the ultrasonic signal asdiscussed herein, vibrational response images of the component 12 can beobtained using, for example, the vibrometer 28. FIG. 5 is a graph withtime on the horizontal axis and amplitude on the vertical axis showingthe waveform sensed by the vibrometer during the duration of the inputpulse. The waveform is separated into five regions, labeled A-E. Each ofthe separate regions A-E were Fourier analyzed, where the Fourieranalysis of region A shows a pure 40 kHz sample vibration. The “bump” inregion B suggests a qualitative change in vibrational behavior. In fact,the analysis shows the presence of a strong sub-harmonic signal at 20kHz, along with all multiples of 20 kHz up to 160 kHz, but with noadditional measurable frequencies. Following the “bump” in region B,region C is a long region where Fourier analysis shows no sub-harmonicspresent, but in which all multiples of 40 kHz are present up to 200 kHz.Thus, in the first three regions A-C, no audible frequencies arepresent.

[0057] A dramatic change in the waveform and in its spectrum occurs inregions D and E, and corresponds to the onset of the audible “screech”.FIG. 6 is a graph with frequency on the horizontal axis and amplitude onthe vertical axis of the Fourier Transform spectrum of region D. As isapparent, region D contains a series of frequencies which are multiplesof {fraction (1/11)}th of the fundamental frequency (40 kHz), togetherwith numerous small, unidentified frequencies.

[0058] In region E, another dramatic switch in the waveform occurs, andthe Fourier Transform becomes a sequence of frequencies that aremultiples of {fraction (1/13)}th of the fundamental frequency (40 kHz),as shown in FIG. 7. In more typical waveforms, there are mixtures ofmany such sequences, involving fractions such as halves, thirds,fourths, fifths, sevenths, eighths, ninths, elevenths, thirteenths,twenty-fourths, etc. There are clear switches to and among sequences inmany of these waveforms where the amplitude increases. Associated withthese increases in amplitude and complexity of the waveform arepronounced increases in heating, as shown in the images. The samephenomenon has been observed using different power supplies,transducers, fundamental frequency, etc.

[0059] The presence of so many frequencies in the vibrational spectrumis clear evidence of quasi-chaotic excitation as described in equations(1)-(3). Equations (1)-(3) were developed on the basis of a harmonicoscillator being “kicked” by another periodic system. This phenomenonhas been observed not only in the case of simple plates, but also withvery large, complex-shaped objects, such as a turbine engine fan disk.Thus, it seems likely that the resonant system here is in fact theacoustic horn and associated electronics, so that it may be instructiveto think not of the horn “kicking” the sample, but rather of the sample“kicking” the horn.

[0060]FIG. 8 is a block diagram of a defect detection system 70 thatgenerates acoustic chaos in an object 72 being tested that may or maynot have a defect. The object 72 is imaged by a thermal imaging camera(not shown), as discussed above, to determine whether a defect exists.In this embodiment, a chaos signal is generated by an electronic chaossignal generator 74 instead of relying on the force applied to theacoustical horn, the coupling material, the coupler thickness and thefrequency and duration of the excitation pulse, as discussed above. Thechaos signal generator 74 can be any device that generates a chaossignal of the type being discussed herein. Generally, the generator 74would include nonlinear circuit elements to create an electricalwaveform that has all of the peculiar frequency components of chaoticsound. Alternately, the chaos signal may be able to be generateddigitally by a digital computer.

[0061] The chaos signal generated by the generator 74 is applied to apower amplifier 76 that amplifies the signal. The amplified chaos signalis applied to a broadband transducer 78. The signal generates a soundsignal in the transducer 78 that is coupled into the object 72 through acoupler 80. Because the signal applied to the transducer 78 is alreadychaotic, it can be linearly coupled into the object 72 by the transducer78. The acoustic signal from the transducer 78 thus induces acousticchaos in the object 72 to increase the heating of the defects in theobject 72.

[0062] According to another embodiment of the present invention, athermography defect detection system excites an object being inspectedwith an ultrasonic excitation signal over multiple frequencies toproduce heat at the location of cracks and crack-like defects in theobject that can be detected by an infrared camera. The object can be anybody comprised of solid materials, such as metals, ceramics, plastics,glasses, coated metals, metal matrix composites, ceramic matrixcomposites and polymer matrix composites.

[0063] As is known in the art, eigen-modes and eigen-frequencies existin an elastic object, which are defined by the object's geometry,elastic properties, additional boundary conditions, such as clamping inthe fixture, and the technique of generating vibrations in the object.The eigen-mode of an object defines the frequency that will resonatewithin the object where vibrations will add. Therefore, the localvibration amplitude in the object, and from this the detectability ofdefects, may significantly depend on the excitation frequency, amplitudeand duration of the excitation signal. An excitation signal with afrequency at or near an eigen-mode of the object results in asubstantial increase of the vibrational amplitude in the object. Becausethe eigen-modes of industrial components are not easily known and canchange as a result of small changes in geometry, elastic properties andboundary conditions of the component, sometimes in a nonlinear manner,the use of a single frequency, amplitude and duration excitation signalmay lead to unpredictable vibration results. Substantial variations inresults have been observed using vibration sources emitting one or morepulses at a predetermined frequency, amplitude or duration.

[0064] Stimulation of the object with a set of frequencies or withchanging frequencies may be advantageous because more than oneeigen-mode in the object can be excited, and therefore the distributionof the vibrations amplitude becomes more even, i.e., the avoidance ofnodes. Particularly, the combination of the different strain amplitudesbelonging to the corresponding frequencies and mode patterns provides anoccurrence of a strain of sufficiently high amplitude and a sufficientnumber of cycles at any site of the object where defects are detected.This can be done by combining different mode patterns with differentnatural frequencies. The possibility to select frequencies is helpful inthe case where special eigen-modes exist that could damage the object,especially thin parts. The excitation signal could be tuned or adjustedto avoid those frequencies.

[0065]FIG. 9 is a block diagram of a thermography defect detectionsystem 90 for detecting defects in an object 92 of the type generallydiscussed herein. The thermography system 90 includes an ultrasonictransducer 94 having a horn 96 that couples sound energy into the object92 at certain defined frequency patterns. In other embodiments, the horn96 can be replaced with a broadband transducer, as will be discussedfurther below. The transducer 94 can be the same as the transducer 14,or another suitable sound instrument consistent with the discussionherein. For example, the transducer 94 can be a piezoelectric, anelectromagnetic or a magneto-strictive element to provide the desiredfrequency patterns. As will be discussed below, the sound energy coupledinto the object 92 is in the form of pulsed frequency signals to heatthe defects (cracks) within the object 92. An infrared camera 98 imagesthe defects that are heated to identify them in the object 92.

[0066] A controller 100 controls the operation of the system 90, andprovides timing between the transducer 94 and the camera 98. Thecontroller 100 controls a signal shaper 102 that provides a signal tothe transducer 94 at the desired pulse rate, pulse duration, frequency,envelope shape, etc., consistent with the discussion herein for thevarious embodiments. The system 90 also includes a vibration sensor 104positioned against the object 92 that listens to the vibrational modesand patterns within the object 92 when it is being excited by theexcitation signal. The vibration sensor 104 can be any sensor suitablefor the purposes discussed herein, such as an accelerometer, an eddycurrent based vibration sensor, an optical vibration sensor, amicrophone, an ultrasonic transducer or an ultrasonic vibration sensor.The sensor 104 provides a signal to the controller 100 indicative of thevibration pattern so that the controller 100 knows what vibrations arebeing induced in the object 92 by the excitation signal. The controller100 can then use this information to change the signal applied to thesignal shaper 102 to vary the excitation signal applied to the object 92from the transducer 94 to get a different, possibly more desirable,vibration pattern within the object 92 to better heat the defects.

[0067] In one embodiment, the transducer 94 is a broad-band transducerthat is able to provide frequencies tuned at different centerfrequencies or a broadband signal having a relatively large frequencyband. The broad-band transducer 94 can provide signals centered atdifferent frequencies sequentially, or at the same time. The frequenciescan be provided in an increasing manner or a decreasing manner,randomly, swept up, swept down, random sweep, etc. Providing multiplefrequency bands may eliminate dead, or unenergized zones, within theobject 92. Also, the excitation signal can be a band of frequencies.Further, the excitation signal can be a chirp-signal whose frequencychanges in time.

[0068] Alternately, the system 90 can employ multiple transducers tunedat different narrow band center frequencies to excite the object 92 withmultiple excitation signals at different frequencies. Thus, the system90 can employ a second transducer 106 that also couples a soundexcitation signal into the object 92, where the transducers 94 and 106would be tuned to different narrow band frequencies. Further, the system90 can employ an array of transducers. The controller 100 would controlthe timing of the excitation signals from the transducers 94 and 106 andthe signal shaper 102 would define the shape of the signals generated bythe transducers 94 and 106 to get the desirable vibrations within theobject 92. Of course, the system 90 could employ more than twotransducers for more than two frequency input signals.

[0069] Flexible excitation systems, applicable to be used for one ormore of the transducer 94, the controller 100 and the signal shaper 102,are known in the art that provide sound and ultrasonic signals. Thesesystems may include suitable arbitrary waveform generators, amplifiers,converters and other related equipment to generate the frequencypatterns. These systems allow the creation of arbitrary or specificallydesigned waveforms composed of selective frequency content and amplitudecharacteristics by the appropriate mixing of continuous signals orcombining of continuous signals and pulse signals, or by specificcontrol of the amplitude of continuous signals. Thus, arbitrary shapesof pulse envelopes and frequency characteristics can be generated. Thesearbitrary shapes can also be generated digitally by a digital computeror by a digital signal shaper. Further, the system components can bedriven dynamically, which allows control of amplitude and frequencybased on additional inputs, such as from vibration sensors andaccelerometers.

[0070] FIGS. 10-18 are graphs showing various excitation signals thatcan be applied to the object 92 having various frequency characteristicsfor various applications. In one embodiment, the various excitationsignals from the transducer 94 are intended to excite the eigen-modes inthe object 92 to further increase or enhance the heating of the defectsin the object 92. In an alternate embodiment, the excitation signalsavoid the eigen-modes in the object 92 to reduce the chance of damagingthe object 92. FIGS. 10-13 only show part of the excitation signal overa 2 ms timeframe. FIG. 15 only shows part of the excitation signal overa 4 ms timeframe. Typical durations of the excitation signal for thesetypes of signals can be about 1 second. FIGS. 14 and 16-19 show thetotal excitation signal.

[0071]FIG. 10 shows an excitation signal pulse that is a combination oftwo frequencies centered at 20 kHz and 21 kHz. FIG. 11 shows anexcitation signal pulse that is a combination of two frequenciescentered at 20 kHz and 40.5 kHz. FIG. 12 shows an excitation signalpulse that is a combination of two frequencies centered at 20 kHz and 41kHz. FIG. 13 shows an excitation signal pulse that is a combination ofthree frequencies centered at 20 kHz, 21 kHz and 22 kHz. FIG. 14 showsan excitation signal pulse that is a Gaussian frequency band around 20kHz. FIG. 15 shows an excitation signal that is a chirp-signal having afrequency sweep upwards. FIG. 16 shows an excitation signal that is asignature signal defined by a set of random pulses in a digital sequence(1, 0, 1, 0, 0, 1, 1, 1, 0, 0, 1, 1 . . . ) that switch the excitationsignal on and off, where the total excitation signal is shown. Therandom set of pulses can be transferred to the sensed infrared signaland decoded to improve the signal-to-noise ratio. FIG. 17 shows anexcitation signal that is based on a rectangular frequency band around20 kHz, where the total excitation signal is shown.

[0072]FIG. 18 shows an excitation signal that has an increasingamplitude with a step at the beginning, where the total excitationsignal is shown. A variation of the amplitude arises if the inducedvibration has to be kept constant by a controller in case of anon-constant transient response of the transducer 94, an unstablecoupling of the transducer 94, an unstable clamping condition, or ifcertain characteristics of the excitation signal, such as an exponentialdecrease, is intended. FIG. 19 shows an excitation signal that is a setof two pulses having a favored envelope frequency. The width of thefirst pulse is small which results in a small thermal diffusion lengthappropriate for detection of surface defects. The second pulse issubstantially wider which results in a larger diffusion lengthappropriate for subsurface defects.

[0073] Various features of the object 92 can be tested, according to theinvention, including the investigation of vibration modes, theperformance of a tuned inspection, and the variation of the envelope ofthe excitation signal (intensity modulation). Investigation of thevibration modes can include performing a frequency sweep of the inputsignal for the determination of natural frequencies and a spatialpattern of eigen-modes of the object 92. Methods of measurement of theobject 92 can include measurement of the phase shift between voltage andcurrent and the effective electric power or vibration amplitude with anadditional sensor.

[0074] For performance of a tuned inspection, variations of thefrequency of the excitation signal can be provided. These variations infrequency include excitation of the object 92 with a set of frequencies,excitation of the object 92 with a frequency band, excitation of theobject 92 with a noise signal, including a frequency band within therange of existing eigen-modes, and excitation of the object 92 with achirp-signal. Repetition of the chirp-signal is possible by the repeatedsweep of the frequency of the excitation signal up and down within adefined band where the eigen-modes exist. Performance of a tunedinspection of the amplitude of the excitation signals, includesproviding the excitation signal with a stepped or varying amplitudepulse or set of pulses, excitation of the object 92 with continuouslyvarying amplitudes in low-to-high or high-to-low in a swept manner, orexcitation of the object 92 with continuously varying amplitudes in acyclic, amplitude manner. Further, the locations of the vibration energyinput based on the eigen-modes of the object 92 can be varied.

[0075] For the variation of the envelope of the excitation signal, theexcitation signal can have a special signature of the envelope, such asrecognition of the signature within the infrared response, such asdiscussed above for FIG. 16. Also, excitation of a signal that favorsspecial frequencies of the envelope, including adaptation to the depthof a defect and thermal properties of the object 92 can be provided asdiscussed above for FIG. 19. These frequencies of the intensitymodulation are typically some orders of magnitude lower than the soundfrequency, as shown in FIG. 19. Also, an excitation signal that variesfrequencies within the operational range can be provided or commerciallyavailable ultrasonic welding devices can be used in various ways. Thefrequency of the excitation signal can be varied, or swept, fromlow-to-high frequencies in the range. Alternately, the frequency of theexcitation signal can be caused to vary in a cyclic manner fromlow-to-high, and from high-to-low, and repeated a number of times in amanner of frequency modulation.

[0076] The excitation signal can keep the vibrational energy transferredinto the object 92 constant in order to balance changes of the couplingand clamping condition based on the measurement of vibration amplitudewith an additional vibration sensor, or on the excitation signal usingan IR response of the object 92 or from a reference sample. Theexcitation signal can have a steadily increasing amplitude, which stopsor is subsequently kept constant, at a level below where damage isexpected. The start of the signal should be at zero amplitude or at asafe amplitude.

[0077] According to another embodiment of the invention, variations ofthe defect detection test using a sequence of N number of excitations,where N is a pre-selected or automatically selected number of excitationpulses greater than one is provided. Each of the excitation pulses from1 to N can be comprised of a pre-selected frequency, amplitude andduration, which is varied from excitation 1 to N in a manner thatresults in different eigen-mode vibrations in the object 92 during eachexcitation interval. The infrared or thermal imaging can remain activeduring the entire N-shot period of time so that defect heating eventsthat are preferential to certain changing vibration conditions can beintegrated or averaged over the entire test sequence.

[0078] The flexible excitation technique will maximize the opportunitiesfor optimum vibration modes which cause a local heating at cracklocations and minimizes arbitrary heating of the object 92 which couldoccur from excessive vibrations during nonlinear vibration mode changes.This combination of maximizing heating from defect locations andminimizing arbitrary or general heating of the object 92 will provideincreased signal-to-noise ratio and aid in identifying indications ofdefects.

[0079] The system 90 can be designed for open-loop or closed-loopcontrol. In the open-loop control embodiment, a tuned envelopeexcitation signal can be used to cause vibrations in the object 92 basedon a predetermined eigen-mode analysis of the object 92, i.e., byanalytical or empirical measurement methods. The predeterminedeigen-modes are evaluated against the characteristics of the signaloptions, and one option is selected for use in the systems test cycle.The characteristics, i.e., frequencies, duration and amplitude, of thetuned or envelope excitation signal can be chosen to control thesensitivity of the test overall, control the levels of stress and straininduced in the object 92 by the vibrations relative to the levelrequired to damage the object 92, control a limited area or areas ofinterest on the object 92, achieve an almost even distribution ofvibration, or select modes that are determined to be effective atheating specific defects of interest for the inspection. In the casewhere the eigen-modes are not exactly known, however, the frequency bandwhere they exist can be identified, and one or more choices of anexcitation signal with a frequency band, noise signal or chirp-signalguarantees that one or more eigen-modes are excited.

[0080] In the closed-loop control embodiment, a tuned excitation signalcan be used to vibrate the object 92. The actual vibrations induced inthe object 92 are measured for the basis of eigen-mode analysis of theobject 92. The analysis can be carried out by computing hardware orsoftware analysis tools, and the results can be used by the thermographysystem 90 to select and control characteristics, i.e., frequencies,duration and amplitude, of the tuned or envelope excitation signal toinduce the appropriate vibrations of the object 92. Thesecharacteristics can be chosen to control the sensitivity of the testoverall, control the levels of stress and strain induced in the object92 by the vibrations relative to the level required to damage the object92, control the limited area or areas of interest on the object 92,achieve an almost even distribution of vibration, or select modes thatare determined to be effective at heating specific defects of interestfor the inspection.

[0081] Ultrasonic vibration exciter devices employing piezoelectricconverters are available in the art that are commonly used forultrasonic welding of plastics and other materials. These devices can beused for the transducer 94. The control system for these devices havesome ability to vary frequency, amplitude, duration and contact forcethrough limited ranges or can be modified internally or by the additionof an input signal conditioner to allow for flexible excitation. Thereare compact, low-cost ultrasonic vibration exciter devices, for example,piezoelectric, electromagnetic or magneto-strictive devices, that areavailable in the art to allow for flexible excitation in configurationsusing known transducing principles for generating signals. Examples ofsuch devices are disclosed in U.S. Pat. Nos. 6,232,701 and 6,274,967.Also, the model F7-1 piezoelectric shaker system manufactured by WilcoxResearch of Gaithersburg, Md. can be used. These devices combined withan arbitrary wave-form generator, flexible function generator ordigitally controlled signal generator, provide an appropriate poweramplifier and microprocessor-based or computer-based control system thatcan be programmed to provide a flexible excitation signal for avibration thermography system as required.

[0082] The availability of compact, low-cost ultrasonic vibrationexciter devices also aids in the application of multiple exciter orarrays of exciters as another implementation. In other words, thetransducer 94 can be replaced with a series of transducers or exciters.Additional flexibilities can be provided to customize the excitationmodes by, for example, selecting combinations of excitercharacteristics, including frequency, duration and amplitude, witheigen-mode features, such as nodes and anti-nodes at selectedfrequencies or combinations of frequencies and vibration modes tooptimize the inspection results for selected areas of interest, types ofdefects and degradation to be indicated in situation variations in theobject 92, such as results from manufacturing variations or frommaterial aging or wear and degradation due to exposure to operationalconditions of the object 92.

[0083] The foregoing discussion discloses and describes merely exemplaryembodiments of the present invention. One skilled in the art willreadily recognize from such discussion, and from the accompanyingdrawings and claims, that various changes, modifications and variationscan be made therein without departing from the spirit and scope of theinvention as defined in the following claims.

What is claimed is:
 1. A defect detection system for detecting a defectin a structure, said system comprising: a sound source for applying asound input signal to the structure, said sound source being coupled tothe structure in a manner so that the sound signal induces acousticchaos in the structure that causes the structure to vibrate in a chaoticmanner and heat the defect; and a thermal imaging camera for generatingthermal images of the structure to identify the heated defect.
 2. Thesystem according to claim 1 wherein a force is applied to the soundsource to couple the sound source to the structure in a manner thatgenerates the acoustic chaos in the structure.
 3. The system accordingto claim 1 further comprising a coupler in contact with the sound sourceand the structure, said sound signal being coupled to the structurethrough the coupler, said coupler being made of a predetermined materialand having a predetermined thickness that act to induce the acousticchaos.
 4. The system according to claim 3 wherein the coupler is anon-linear coupler.
 5. The system according to claim 3 wherein thecoupler is selected from the group consisting of copper, automotivegasket material, leather, duct tape, Teflon, paper products and cork. 6.The system according to claim 1 wherein the sound source includes achaos signal generator and a transducer, said chaos signal generatorgenerating a chaos signal that is applied to the transducer, saidtransducer causing the structure to vibrate in a chaotic manner.
 7. Thesystem according to claim 1 wherein the sound source includes anultrasonic transducer, said ultrasonic transducer including a transducerhorn that is coupled to the structure, and wherein the sound inputsignal generated by the ultrasonic transducer causes the transducer hornto impact against the structure.
 8. The system according to claim 1wherein the sound source includes an electromagnetic acoustictransducer.
 9. The system according to claim 1 further comprising adevice for determining vibrations of the structure in response to thesound input signal.
 10. The system according to claim 9 wherein thedevice is a doppler laser vibrometer.
 11. The system according to claim9 wherein the device is a microphone.
 12. The system according to claim1 wherein the acoustic chaos is defined by a range of frequenciesproviding a vibrational waveform whose spectral content is related tothe frequency of the sound input signal by ratios of rational numbers.13. A system for creating acoustic chaos in a structure, said systemcomprising a sound source coupled to the structure under a predeterminedforce, said sound source applying a pulsed sound signal to thestructure, wherein the amount of force, the duration of the pulsed soundsignal and the frequency of the sound signal act to induce acousticchaos in the structure and cause the structure to vibrate in a chaoticmanner.
 14. A defect detection system for detecting a defect in astructure, said system comprising: an electronic chaos signal generatorfor generating a chaos signal; a broadband transducer responsive to thechaos signal from the chaos signal generator; and a coupler coupling thetransducer to the structure, wherein the transducer converts the chaossignal to a sound signal that is applied to the structure through thecoupler, wherein the sound signal induces acoustic chaos in thestructure that acts to heat the defect.
 15. A defect detection systemfor detecting defects in a structure, said system comprising: a soundsource for applying a sound input signal to the structure, said inputsignal including a plurality of frequency signals having differentfrequencies, said input signal heating the defects in the structure; anda thermal imaging camera for generating thermal images of the structureto identify the defects.
 16. The system according to claim 15 whereinthe input signal is a combination of two or more frequency signalscentered at different frequencies.
 17. The system according to claim 15wherein the input signal has a Gaussian frequency band.
 18. The systemaccording to claim 15 wherein the input signal is a chirp-signal whosefrequency changes in time.
 19. The system according to claim 15 whereinthe input signal is a signature signal having a set of pseudo-randompulses.
 20. The system according to claim 15 wherein the input signalhas an increasing, decreasing or constant amplitude in varioussequential combination, including optionally steps in amplitude.
 21. Thesystem according to claim 15 wherein the input signal is based on arectangular frequency band.
 22. The system according to claim 15 whereinthe input signal is a favored envelope frequency including one pulsehaving a small pulse width and another pulse having a larger pulse widthfor detection of defects in different depths.
 23. The system accordingto claim 15 wherein the sound source is selected from the groupconsisting of EMATs, ultrasonic vibrators, piezoelectric vibrators,electromagnetic vibrators and magneto-strictive vibrators.
 24. Thesystem according to claim 15 further comprising a signal shaper, saidsignal shaper generating the input signal to have a predeterminedduration, amplitude and frequency.
 25. The system according to claim 15wherein the sound source is a broadband transducer capable of providinga broad-band frequency signal.
 26. The system according to claim 15wherein the sound source includes a plurality of transducers each beingtuned to a different narrow band center frequency.
 27. The systemaccording to claim 15 further comprising a vibration sensor coupled tothe structure, said vibration sensor sensing vibrations in thestructure.
 28. The system according to claim 27 wherein the vibrationsensor is selected from the group consisting of an eddy current basedvibration sensor, an accelerometer, an optical vibration sensor, amicrophone, an ultrasonic transducer and an ultrasonic vibration sensor.29. The system according to claim 27 wherein the vibration sensormeasures a phase shift between current and voltage of the sensedvibrations to determine the natural frequencies of the structure. 30.The system according to claim 27 wherein the vibration sensor measuresan amplitude characteristic of current or voltage of the sensedvibrations to determine the natural frequencies of the structure. 31.The system according to claim 15 wherein the input signal is a tunedexcitation signal that provides an open-loop or a closed loop control.32. A defect detection system for detecting defects in a structure, saidsystem comprising: a sound source for applying a sound input signal tothe structure, said input signal having one or more frequencies or asingle frequency signal with an amplitude modulation selected to be ator near an eigen-mode of the structure or selected to avoid theeigen-mode of the structure, said input signal heating the defects inthe structure; and a thermal imaging camera for generating thermalimages of the structure to identify the defects.
 33. The systemaccording to claim 32 wherein the input signal is selected from thegroup of input signals consisting of an input signal having acombination of two or more frequency signals centered at differentfrequencies, an input signal that has a Gaussian frequency band, aninput signal that is a chirp-signal, an input signal that is a signaturesignal having a set of random pulses, an input signal that has arectangular frequency band, an input signal that has an increasingamplitude with a step, and an input signal that includes one pulsehaving a short pulse duration and another pulse having a wide pulseduration for detection of defects in different depths.
 34. A defectdetection system for detecting defects in a structure, said systemcomprising: a sound source for applying a sound input signal to thestructure, said input signal being a single frequency signal with anamplitude modulation, said input signal heating the defects in thestructure; and a thermal imaging camera for generating thermal images ofthe structure to identify the defects.